EP0989644B1 - Quantum cascade light emitter with pre-biased internal electronic potential - Google Patents
Quantum cascade light emitter with pre-biased internal electronic potential Download PDFInfo
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- EP0989644B1 EP0989644B1 EP99307257A EP99307257A EP0989644B1 EP 0989644 B1 EP0989644 B1 EP 0989644B1 EP 99307257 A EP99307257 A EP 99307257A EP 99307257 A EP99307257 A EP 99307257A EP 0989644 B1 EP0989644 B1 EP 0989644B1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
- H01S5/3402—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers intersubband lasers, e.g. transitions within the conduction or valence bands
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
Definitions
- This invention relates generally to superlattice (SL) semiconductor devices and, more particularly, to quantum cascade (QC) SL semiconductor light emitters (e.g.,lasers).
- SL superlattice
- QC quantum cascade
- Semiconductor SLs are multi-layered structures with unique electronic properties. These structures comprise a periodic stack of alternating thin (e.g., nanometer thick) layers of two different semiconductor materials having different bandgaps (i.e., lower bandgap quantum well (QW) layers interleaved with wider bandgap barrier layers). As described by G. Scamarcio et al., Science , Vol. 276, pp. 773-776 (March 1997), the period of this structure ( ⁇ 5 nm) is typically much larger than the lattice constant of the bulk crystalline constituents ( ⁇ 0.5 nm).
- QW quantum well
- minibands This superimposed potential splits the conduction and valence bands into a series of much narrower bands (typically tens to a few hundred millielectron volts wide in the strong tunnel-coupling regime) called minibands , which are separated by energy gaps ( minigaps ) along the direction perpendicular to the layers.
- minibands energy gaps along the direction perpendicular to the layers.
- To form a miniband requires that the wavefunctions of the states in each of the QWs are delocalized; i.e., the wavefunctions extend over many QWs, thus indicating that the QWs are strongly coupled to one another; they are not localized in which case the QWs would be effectively decoupled from one another.
- SLs their long inter-miniband relaxation time compared to the intra-miniband one, has been used to develop an SL QC laser with intrinsic population inversion and very large current carrying capabilities and optical power outputs.
- active region repeat units radiatative transition (RT) regions plus interleaved injection/relaxation (I/R) regions
- RT radiation transition
- I/R interleaved injection/relaxation
- a light emitter according to the invention is as set out in claim 1. Preferred forms are set out in the dependent claims.
- pre-bias instead of trying to keep the SLs of a QC laser field free, we "pre-bias" the actual electronic potential by varying the SL period (and hence average composition) so as to achieve an essentially flat profile, on average, of upper and lower minibands, despite the presence of an applied field in the SLs.
- pre-bias will be more fully developed hereinafter.
- the thicknesses of the QW layers are varied from QW layer to QW layer so as to increase in the direction of the applied field.
- the upper and lower lasing levels are located, in the absence of an applied electric field, each at different energies from layer to layer within the first subset, so that despite the presence of an applied field, the desired flatband condition of the upper and lower minibands is realized.
- the thicknesses of the QW layers within the first subset are varied from QW layer to QW layer so as to increase in the direction of the applied field.
- the thicknesses of a second subset of the barrier layers are also varied from barrier layer to barrier layer within a second subset so as to vary (either increase or decrease) in the direction of the applied field.
- a method of making a QC SL light emitter comprises the steps of: (1) fabricating a multiplicity of SL unipolar RT regions interleaved with I/R regions, each RT region including a plurality of QW layers interleaved with barrier layers, the QW layers having energy states characterized by upper and lower minibands, and (2) in at least a first subset of the QW layers, increasing the thicknesses of the QW layers from QW layer to QW layer in a first direction transverse to the layers.
- An alternative embodiment includes the additional step of, in at least a second subset of barrier layers, varying (decreasing or increasing) the thicknesses of the barrier layers from barrier layer to barrier layer in the same direction, so that despite the presence of an applied field within the SLs, the desired flatband condition of the upper and lower minibands is realized.
- FIGs. 1-4 have not been drawn to scale.
- the symbol A stands for Angstroms
- the symbol A stands for Amperes.
- a QC SL semiconductor light emitter (e.g., laser) 10 comprises a QC SL active region 14 sandwiched between an upper cladding region 16 and a substrate 12 which serves as a lower cladding region.
- a lower cladding region separate from the substrate, may be formed between the substrate and the active region.
- the upper cladding region 16 is illustratively formed in the shape of a mesa or trapezoid typical of ridge waveguide laser structures.
- the mesa may be shallow-etched as shown to stop at the active region 14, or it may be deep-etched so that the mesa extends into the active region.
- an electrically insulating layer 18 (e.g., Si 3 N 4 or SiO 2 ) is formed over the top of the device and is patterned to form an opening which exposes a portion of the top of the mesa.
- a first electrode 20 is formed over the insulating layer 18 and in the opening so as to contact the upper cladding region (usually by means of a highly doped contact-facilitating layer, not shown), and a second electrode 22 is formed on the substrate 12.
- the substrate itself may be a single crystal semiconductor body or a combination of such a body with another layer (e.g., an epitaxial layer grown on the top surface of the body).
- lasers of this type are fabricated from Group III-V compound semiconductors; e.g., In-based Group III-V compounds such as GaInAs and AlInAs.
- Suitable optical feedback is typically provided by an optical cavity resonator formed, for example, by cleaved crystal facets, distributed feedback (DFB)gratings, and distributed Bragg reflectors (DBRs), or a combination of them.
- DFB distributed feedback
- DBR distributed Bragg reflector
- the active region 14 includes a multiplicity of N repeat units, each unit comprising a "pre-biased" SL RT region 14.1 and an I/R region 14.2.
- the effect of "pre-biasing" the internal electronic potential of the SLs is to produce a flatband condition of upper and lower minibands despite the presence of the electric field in the SLs produced by the applied bias voltage.
- the lower miniband 14.3 of one RT region 14.1 is aligned with the upper miniband 14.4 of an adjacent RT region 14.5, with the two aligned minibands being matched to and bridged by a single miniband 14.6 of the intervening I/R region 14.2.
- lasing is an inter-miniband process; i.e., as shown by the wavy arrow 15, stimulated emission at a wavelength ⁇ takes place in RT region 14.1 between the bottom energy level 2 of the upper miniband 14.7 and the top energy level 1 of the lower miniband 14.3.
- ⁇ takes place in RT region 14.1 between the bottom energy level 2 of the upper miniband 14.7 and the top energy level 1 of the lower miniband 14.3.
- lasing also takes place at the same wavelength in RT region 14.5 between the bottom energy level 2 of upper miniband 14.4 and top energy level 1 of lower miniband 14.8.
- the wavefunctions (moduli squared) of the levels 1 and 2 are also shown in FIG. 2. Note that within each RT these wavefunctions extend across all of the QWs of that RT, thus indicating that the QWs are effectively coupled to one another.
- the internal electronic potential of an RT region is pre-biased in the sense that an actual potential is built into the device; i.e., the energy levels of at least a first subset of the QW layers of the RT region are staggered from QW layer to QW layer.
- "pre-biasing" is achieved by monotonically increasing the thicknesses of these QW layers in a first direction transverse to the layers; e.g., in the direction of the applied electric field (generated by the bias voltage).
- the thicknesses of a second subset of the barrier layers may also be varied form barrier layer to barrier layer.
- the thicknesses of these barrier layers monotonically decrease or increase in the same first direction.
- subset is intended to embrace fewer than all of the QW (or barrier) layers in the RT region or all of the QW (or barrier) layers in the RT region (i.e., the complete set). Moreover, the first and second subsets need not embrace the same group of layers and need not contain the same number of layers.
- FIG. 3 The desired flatband condition of the minibands 30,40 in an SL in the presence of an applied field E A is depicted in FIG. 3.
- the SL is shown as a typical prior art, field-compensated periodic structure in which each QW layer has the same thickness t w and each barrier layer has the same thickness t b .
- each miniband of FIG. 3 would be split into separate quantum states at different energies in each quantum well.
- an SL is "pre-biased" by increasing the QW layer thickness t w in the direction of the electric field E A (shown in phantom since it would be applied only during the operation of the laser).
- t w4 ⁇ t w3 ⁇ t w2 ⁇ t w1 which means that the quantum states in the narrower QWs are at higher energies than the corresponding states in the wider QWs, and that the energy difference between the upper and lower levels in the narrower QWs is larger than in the wider QWs. Therefore, by grading the thicknesses of the QWs we are able to stagger the quantum states from QW to QW in such a way that the desired flatband condition of the upper and lower minibands is realized.
- a desired flat miniband condition of upper and lower minibands can be achieved by varying only the QW thicknesses as previously described.
- the QW and barrier layer thicknesses that is, in accordance with another embodiment of our invention, we preferably also vary (increase or decrease) the barrier thickness the direction of the applied field. Whether or not the thicknesses of a particular subset of barrier layers is increased or decreased is determined empirically (including the use of computer modeling programs). Objectives of varying tb include ensuring adequate coupling of the QWs, facilitating electron transport between the QWs, and providing relatively high oscillator strength (i.e., dipole matrix element z 21 ). In any case, the period defined by (t w + t b ) should not vary so widely that the SL characteristics of the RT regions are significantly impaired.
- This example describes a Group III-V compound semiconductor, QC "prebiased" SL laser in accordance with one embodiment of our invention.
- Various materials, dimensions and operating conditions are provided by way of illustration only and, unless otherwise expressly stated, are not intended to limit the scope of the invention.
- the term undoped means that a particular semiconductor layer or region is not intentionally doped; i.e., any doping of such a region or layer is relatively low and typically results from residual or background doping in the chamber used to grow the layers of the device.
- the schematic conduction band diagram of the lasers is shown in FIG. 2.
- Two well-defined minibands 14.3 and 14.4 were obtained in the adjacent "pre-biased" SL RT regions 14.1 and 14.5, respectively, matched to a single I/R miniband 14.6 which bridges them together across the cascading stages.
- GaInAs/AlInAs samples were grown by molecular beam epitaxy (MBE) lattice-matched to a low 5 doped InP substrate.
- MBE molecular beam epitaxy
- Table I The complete structure, shown in Table I, included a core sandwiched between the substrate and an upper cladding region but separated therefrom by well known GaInAs/AlInAs digitally graded transition layers.
- the core included the 28 repeat units bounded by relatively thick, low doped GaInAs layers.
- a thin, highly doped GaInAs contact-facilitating layer was formed on top of the upper cladding region.
- the substrate itself served as a lower cladding region.
- the wafers were processed into mesa-etched ridge waveguide lasers of several widths and cleaved into bars typically about 2-3 mm long, with the laser output facets left uncoated. In some applications, however, it may be desirable to coat the facets, as is well known in the art.
- I/R Region Composition Doping Concentration (cm -3 ) Thickness (nm) GaInAs undoped 2.3 AlInAs undoped 2.5 GaInAs 2 ⁇ 10 17 2.3 AlInAs 2 ⁇ 10 17 2.5 GaInAs 2 ⁇ 10 17 2.2 AlInAs 2 ⁇ 10 17 2.6 GaInAs 2 ⁇ 10 17 2.0 AlInAs 2 ⁇ 10 17 2.6 GaInAs undoped 1.9 AlInAs undoped 2.7 GaInAs undoped 1.9 AlInAs undoped 2.9 GaInAs undoped 1.8 AlInAs undoped 3.5
- Table III shows the varying layer thicknesses of the "pre-biased" RT regions for both samples D2369 and D2404, in accordance with one embodiment of our invention.
- the applied electric field (not shown) would be oriented in the upward direction; i.e., from the bottom to the top of table.
- the thicknesses of the six GaInAs QW layers increased monotonically from 3.5 to 5.1 nm (35 ⁇ to 51 ⁇ ) in the direction of the field.
- the subset of graded QW layers is the complete set, but as noted before the subset could include fewer than all of the QW layers in each RT region.
- the thicknesses of only the bottom most subset of four AlInAs barrier layers is varied, in this case monotonically decreased in the direction of the field as follows: 2.5 nm (25 ⁇ ), 1.3 nm (13 ⁇ ), 1.2 nm (12 ⁇ ), 1.1 nm (11 ⁇ ).
- the bottom most 2.5 nm barrier could be thinner (e.g., 1.4 nm (14 ⁇ )), but was optionally designed thicker so as provide a degree of decoupling of the RT region from the adjacent I/R region.
- the topmost subset of four barrier layers all had a thickness of 1.1 nm (11 ⁇ ).
- the emitted peak power in pulsed operation of a laser from sample D2369 is shown in FIG. 5 as a function of drive current.
- the ridge waveguide was etched deeply through the 28 stages in order to have a well-defined measure of the active device size and current density. Peak powers of almost 900 mW at 5 K and nearly 300 mW at room temperature were obtained.
- the voltage at threshold ( ⁇ 8.5 V) corresponds well to the electric field for which the structure was designed.
- the threshold current density at 5 K, J th 1.5 kA/cm, was almost a factor of three better than that of any previously published inter-miniband SL laser (See, Tredicucci et al., supra.) and is comparable to that of conventional inter-subband QC lasers of similar wavelength of operation (See, C. Gmachl et al., Appl. Phys. Lett., Vol. 72, No. 24, pp. 3130-3132 (June 1998)).
- the measured dependence of the threshold current density on temperature is plotted in the inset to FIG. 6 for sample D2369.
- the finite temperature behavior was estimated by adding to the well known formula for J th of QC lasers a term that takes into account the thermally activated population of the lower lasing level (state 1).
- the threshold current densities so computed are also plotted in the inset to FIG. 6.
- the agreement with our experimental results is good, considering the uncertainties of the model.
- the low threshold currents of our lasers are particularly desirable for room temperature operation at high repetition rates, where heat dissipation becomes an issue.
- this property combined with the peak power performance, results in a 14 mW average power achieved at 295 K from a laser obtained from sample D2404. This power was measured with near-unity collection efficiency using a parabolic cone and a conventional thermopile power meter.
- Devices from sample D2404 were also processed in a broad-area configuration as shown in FIG. 1 (rather than as deep etched, narrower mesa, ridge waveguides) by limiting the etching procedure to the cladding layers. These lasers outperformed the standard deep-etched lasers in terms of peak power, as shown in FIG. 7, where the L-I characteristics at various temperatures of two such devices are shown. The value of 0.5 W was obtained at room temperature. The room temperature multi-mode spectrum of the emission of our broad area lasers just above threshold is shown in the inset of FIG. 7.
- the limiting factor in the continuous wave (CW) operation of inter-subband and inter-miniband lasers originates from the large power dissipation of the devices. Due to the finite thermal resistance of the lasers, the temperature of the active region is much higher than that of the heat sink.
- the substrate temperature T sub is a good approximation of the latter, and a typical maximum T sub for CW operation (T sub,max ) is about 145 K.
- the CW optical power as a function of drive current of one device from wafer D2433 is shown in FIG. 8 for various temperatures. More than 300 mW of optical power was obtained at 25 K, whereas 20 mW was still obtained at 150 K. The laser ceased to operate in a CW mode at a high temperature of 160 K. As shown in the inset of FIG. 8, the CW spectrum at a cryogenic temperature just above threshold was single mode at a center wavelength of about 7.22 ⁇ m, in excellent agreement with the computed energy difference between the upper and lower inter-minibands. The higher values of the slope efficiency were obtained for the samples with more than 19 stages, according to the well-known cascading scheme in which each electron injected above threshold emits one photon per stage.
- a slope efficiency of 478 mW/A was recorded for a 28 stage laser (from wafer D2404) which was 3 mm long and 16.7 um wide. This measured efficiency compared very well with a calculated value 472 mW/A. The resulting external differential quantum efficiency of 5.6 was greater than unity, which is possible only in laser with cascaded active regions.
- the variations built into the SL e.g., of QW or barrier layer thickness
- the variations built into the SL need not be linear changes in a device parameter, nor do they have to be monotonic changes.
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Applications Claiming Priority (2)
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US159127 | 1998-09-23 | ||
US09/159,127 US6055254A (en) | 1998-09-23 | 1998-09-23 | Quantum cascade light emitter with pre-biased internal electronic potential |
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EP0989644A1 EP0989644A1 (en) | 2000-03-29 |
EP0989644B1 true EP0989644B1 (en) | 2005-03-30 |
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EP99307257A Expired - Lifetime EP0989644B1 (en) | 1998-09-23 | 1999-09-14 | Quantum cascade light emitter with pre-biased internal electronic potential |
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US (1) | US6055254A (ja) |
EP (1) | EP0989644B1 (ja) |
JP (1) | JP3522606B2 (ja) |
DE (1) | DE69924439T2 (ja) |
Families Citing this family (20)
Publication number | Priority date | Publication date | Assignee | Title |
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US6563852B1 (en) | 2000-05-08 | 2003-05-13 | Lucent Technologies Inc. | Self-mode-locking quantum cascade laser |
US6560259B1 (en) | 2000-05-31 | 2003-05-06 | Applied Optoelectronics, Inc. | Spatially coherent surface-emitting, grating coupled quantum cascade laser with unstable resonance cavity |
US6463088B1 (en) * | 2000-07-07 | 2002-10-08 | Lucent Technologies Inc. | Mesa geometry semiconductor light emitter having chalcogenide dielectric coating |
EP1195865A1 (fr) * | 2000-08-31 | 2002-04-10 | Alpes Lasers SA | Laser à cascades quantiques |
US6556604B1 (en) * | 2000-11-08 | 2003-04-29 | Lucent Technologies Inc. | Flat minibands with spatially symmetric wavefunctions in intersubband superlattice light emitters |
DE10061234C2 (de) * | 2000-12-08 | 2003-01-16 | Paul Drude Inst Fuer Festkoerp | Unipolarer Halbleiterlaser ohne Injektionsschichten |
US20020097471A1 (en) * | 2001-01-22 | 2002-07-25 | Bethea Clyde George | Data transmission via direct modulation of a mid-IR laser |
EP1249905A1 (fr) * | 2001-04-10 | 2002-10-16 | Alpes Lasers SA | Laser semi-conducteur muni d'un miroir |
JP4576086B2 (ja) * | 2001-05-23 | 2010-11-04 | 明広 石田 | 光機能性化合物半導体超格子構造物の製造方法および光機能性多層体の製造方法 |
US6728282B2 (en) | 2001-06-18 | 2004-04-27 | Lucent Technologies Inc. | Engineering the gain/loss profile of intersubband optical devices having heterogeneous cascades |
US20040208602A1 (en) * | 2001-12-01 | 2004-10-21 | James Plante | Free space optical communications link tolerant of atmospheric interference |
US6788727B2 (en) * | 2002-06-13 | 2004-09-07 | Intel Corporation | Method and apparatus for tunable wavelength conversion using a bragg grating and a laser in a semiconductor substrate |
US20040109692A1 (en) * | 2002-12-09 | 2004-06-10 | James Plante | FSO communication systems having high performance detectors |
US7010010B2 (en) * | 2003-06-19 | 2006-03-07 | Lucent Technologies, Inc. | Broadband cascade light emitters |
JP4250573B2 (ja) * | 2004-07-16 | 2009-04-08 | キヤノン株式会社 | 素子 |
JP5641667B2 (ja) * | 2007-01-18 | 2014-12-17 | 浜松ホトニクス株式会社 | 量子カスケードレーザ |
JP5106260B2 (ja) * | 2008-06-16 | 2012-12-26 | キヤノン株式会社 | カスケードレーザ素子 |
JP5256268B2 (ja) * | 2010-10-21 | 2013-08-07 | シャープ株式会社 | 太陽電池 |
US8948226B2 (en) * | 2012-08-20 | 2015-02-03 | The Board Of Trustees Of The University Of Illinois | Semiconductor device and method for producing light and laser emission |
US9088126B2 (en) | 2013-10-17 | 2015-07-21 | The Trustees Of Princeton University | Single-mode quantum cascade lasers with enhanced tuning range |
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US5509025A (en) * | 1994-04-04 | 1996-04-16 | At&T Corp. | Unipolar semiconductor laser |
US5457709A (en) * | 1994-04-04 | 1995-10-10 | At&T Ipm Corp. | Unipolar semiconductor laser |
US5502787A (en) * | 1995-05-22 | 1996-03-26 | At&T Corp. | Article comprising a semiconductor waveguide structure |
US5727010A (en) * | 1996-03-20 | 1998-03-10 | Lucent Technologies Inc. | Article comprising an improved quantum cascade laser |
JP3159946B2 (ja) * | 1996-11-06 | 2001-04-23 | ルーセント テクノロジーズ インコーポレイテッド | 量子カスケードレーザを有する物品 |
US5936989A (en) * | 1997-04-29 | 1999-08-10 | Lucent Technologies, Inc. | Quantum cascade laser |
-
1998
- 1998-09-23 US US09/159,127 patent/US6055254A/en not_active Expired - Lifetime
-
1999
- 1999-09-14 DE DE69924439T patent/DE69924439T2/de not_active Expired - Lifetime
- 1999-09-14 EP EP99307257A patent/EP0989644B1/en not_active Expired - Lifetime
- 1999-09-22 JP JP26801999A patent/JP3522606B2/ja not_active Expired - Lifetime
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JP3522606B2 (ja) | 2004-04-26 |
DE69924439T2 (de) | 2006-02-16 |
US6055254A (en) | 2000-04-25 |
EP0989644A1 (en) | 2000-03-29 |
JP2000101201A (ja) | 2000-04-07 |
DE69924439D1 (de) | 2005-05-04 |
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